Rear End Wiggling Around While Braking

[racer]

I still think I disagree with your explanation, or perhaps terminology, of pre-load and suspension; but, until I have more time to sort through and get a better grip on what you are saying above, I will refrain from further comment at this time.

 

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OK...

 

If one is compressing the spring against the 'top out stop' when adjusting the pre-load (at static sag = 0), like you say, then one is effectively decreasing the distance the suspension can or will compress from that point (topped out) and the spring will only ever extend as far as that point where you have compressed it against the 'top out stop'. Pre-loaded, the spring is now partially compressed (at 'topped out') and has less distance to travel until it binds or 'bottoms out'. Neither it, nor the shock, will extend more as the shock is 'topped out' and the spring can only compress further from that point of partial compression known as 'pre-loaded'.

 

In fact, if you added more pre-load while riding in the middle of a corner under some compression, you would effectively shorten the travel even more as you brought the shock body up from the spring and closer to the 'top out stop' for that given compression (thereby also increasing ride height for that given compression). But, the travel decreases.

 

Please refer to the lovely enlarged photo of a Penske racing shock w/Hyperpro spring installed here:

 

http://en.wikipedia.org/wiki/Image:Penske_shock.jpg

 

(Anyone know the color code on that spring? lol)

 

The shock in the photo is topped out as the pressure of the spring is applying force to the collar. No matter how much you compress the spring, the shock will never extend any further than it is in the photo. Topped out is topped out. And pre-loaded is pre-loaded. The (compressed) spring is what dictates the travel (even if you hit the bump stop/bottom stop on the shock before binding the spring). Full stop.

 

 

Otherwise, my spring would flop around on the shock every time I did a stoppie or even lifted the rear wheel under hard braking. That is the whole point of the zero/negative pre-load as it allows the shock to lift off of the spring (assuming a spring shorter than the travel at top out).

[racer]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

The only way to change the spring rate is to change out the spring. Preload simply changes the amount of sag. This is true as long as you are working anywhere within the safe and relevant preload adjustment range. When you adjust preload you are just changing the position of the spring relative to the top-out and bottom-out points, which changes how much the suspension will sag under your weight and the bikes weight. It should be called a sag adjustment so people would stop being so confused about it.

 

 

Your understanding or explanation of what pre-load is or does seems flawed here.

 

When you adjust or increase pre-load you are compressing the spring and making it shorter by "loading" the spring prior (or pre-) to putting your butt on the bike. You are not "just changing the position of the spring relative to the top-out and bottom-out points".

 

The comment that "preload simply changes the amount of sag" is oversimplified. So now, in order to try and explain it, I'll write a bunch of nonsense that probably nobody cares about and only makes sense to me.

 

If we adjust the preload while the suspension/shock is topped out, then yes we are compressing the spring against the top out stop. But, we don't ride around with our suspension topped out all the time, most of the time it is compressed some amount. And perhaps the most important moment for suspension is in the middle of a turn to keep the tires on the ground at maximum lean angle, and in this moment both front and rear suspension are compressed.

 

So lets evaluate how a preload adjustment would affect the bike while the suspension is sagging or compressed and not topped out. Let's say we get on our bike and while sitting on causing the suspension to sag, a friend cranks down on the preload, the spring stays compressed under the weight of the rider and bike, the amount of weight on the spring is not changing, thus, the amount of compression of the spring itself is not changing. Thus, in this scenario, we are NOT actually compressing the spring, it is just staying compressed under the weight of bike and rider, and we are just moving it down the shock, or in other words moving it closer to the top out point and further away from the bottom out point.

 

No. It does not move the spring further from the bottom out point, the spring and suspension remain exactly the same wrt the bottom out point. The shock body moves closer to the top out point for the same spring compression and spring compression is the ONLY factor determining how close to the bottom out point the suspension is.

[harnois]

I still think I disagree with your explanation, or perhaps terminology, of pre-load and suspension; but, until I have more time to sort through and get a better grip on what you are saying above, I will refrain from further comment at this time.

 

--------

 

OK...

 

If one is compressing the spring against the 'top out stop' when adjusting the pre-load (at static sag = 0), like you say, then one is effectively decreasing the distance the suspension can or will compress from that point (topped out) and the spring will only ever extend as far as that point where you have compressed it against the 'top out stop'. Pre-loaded, the spring is now partially compressed (at 'topped out') and has less distance to travel until it binds or 'bottoms out'. Neither it, nor the shock, will extend more as the shock is 'topped out' and the spring can only compress further from that point of partial compression known as 'pre-loaded'.

 

In fact, if you added more pre-load while riding in the middle of a corner under some compression, you would effectively shorten the travel even more as you brought the shock body up from the spring and closer to the 'top out stop' for that given compression (thereby also increasing ride height for that given compression). But, the travel decreases.

 

Please refer to the lovely enlarged photo of a Penske racing shock w/Hyperpro spring installed here:

 

http://en.wikipedia.org/wiki/Image:Penske_shock.jpg

 

(Anyone know the color code on that spring? lol)

 

The shock in the photo is topped out as the pressure of the spring is applying force to the collar. No matter how much you compress the spring, the shock will never extend any further than it is in the photo. Topped out is topped out. And pre-loaded is pre-loaded. The (compressed) spring is what dictates the travel (even if you hit the bump stop/bottom stop on the shock before binding the spring). Full stop.

 

 

Otherwise, my spring would flop around on the shock every time I did a stoppie or even lifted the rear wheel under hard braking. That is the whole point of the zero/negative pre-load as it allows the shock to lift off of the spring (assuming a spring shorter than the travel at top out).

 

I understand the shock in the pic is topped out and will never get any longer no matter what you do with the preload. Top out is top out, like you said, it will not change. But bottom out will not change due to a preload adjustment either, thus available travel has not changed. The definition of bottomed out is that the shock has hit that bump stop/bottom stop and can not compress any further. That WILL happen before the spring itself bottoms out, as long as you are in the intended preload adjustment range. Another way to look at it is that the amount of shock shaft (the polished chromy part that disappears into the shock body when the shock is compressed) showing when topped out represents the amount of travel of the shock. No matter what you do with the preload you'll see the same length of shiny shaft at the bottom of the shock when it's topped out.

 

The (compressed) spring is what dictates the travel (even if you hit the bump stop/bottom stop on the shock before binding the spring).

 

If you hit the bump stop/bottom stop, didn't the bump stop/bottom stop just dictate the travel?

[harnois]

...

 

Pre-load will effectively increase the spring rate (essentially at the expense of geometry and suspension travel).

 

The only way to change the spring rate is to change out the spring. Preload simply changes the amount of sag. This is true as long as you are working anywhere within the safe and relevant preload adjustment range. When you adjust preload you are just changing the position of the spring relative to the top-out and bottom-out points, which changes how much the suspension will sag under your weight and the bikes weight. It should be called a sag adjustment so people would stop being so confused about it.

 

 

Your understanding or explanation of what pre-load is or does seems flawed here.

 

When you adjust or increase pre-load you are compressing the spring and making it shorter by "loading" the spring prior (or pre-) to putting your butt on the bike. You are not "just changing the position of the spring relative to the top-out and bottom-out points".

 

The comment that "preload simply changes the amount of sag" is oversimplified. So now, in order to try and explain it, I'll write a bunch of nonsense that probably nobody cares about and only makes sense to me.

 

If we adjust the preload while the suspension/shock is topped out, then yes we are compressing the spring against the top out stop. But, we don't ride around with our suspension topped out all the time, most of the time it is compressed some amount. And perhaps the most important moment for suspension is in the middle of a turn to keep the tires on the ground at maximum lean angle, and in this moment both front and rear suspension are compressed.

 

So lets evaluate how a preload adjustment would affect the bike while the suspension is sagging or compressed and not topped out. Let's say we get on our bike and while sitting on causing the suspension to sag, a friend cranks down on the preload, the spring stays compressed under the weight of the rider and bike, the amount of weight on the spring is not changing, thus, the amount of compression of the spring itself is not changing. Thus, in this scenario, we are NOT actually compressing the spring, it is just staying compressed under the weight of bike and rider, and we are just moving it down the shock, or in other words moving it closer to the top out point and further away from the bottom out point.

 

No. It does not move the spring further from the bottom out point, the spring and suspension remain exactly the same wrt the bottom out point. The shock body moves closer to the top out point for the same spring compression and spring compression is the ONLY factor determining how close to the bottom out point the suspension is.

 

Perhaps "move the spring" is just not a good way for me to describe it. I'm going to define a term here in case it's not already clear what I mean by it:

 

Sag point: the position of the suspension when it is sagging under the weight of bike and rider. So if we have 1 inch of sag the the sag point is 1 inch from top out.

 

Perhaps instead of move the spring, I should say, cranking down on the preload moves the sag point closer to top out point and further from bottom out point. Bottom line though is, if a bike has 4.7 inches of travel as specified then it has 4.7 inches of travel regardless of prelaod adjustment - as long as we are within the intended preload adjustment range.

[racer]

I understand the shock in the pic is topped out and will never get any longer no matter what you do with the preload. Top out is top out, like you said, it will not change. But bottom out will not change due to a preload adjustment either, thus available travel has not changed. The definition of bottomed out is that the shock has hit that bump stop/bottom stop and can not compress any further. That WILL happen before the spring itself bottoms out, as long as you are in the intended preload adjustment range. Another way to look at it is that the amount of shock shaft (the polished chromy part that disappears into the shock body when the shock is compressed) showing when topped out represents the amount of travel of the shock. No matter what you do with the preload you'll see the same length of shiny shaft at the bottom of the shock when it's topped out.

 

The (compressed) spring is what dictates the travel (even if you hit the bump stop/bottom stop on the shock before binding the spring).

 

If you hit the bump stop/bottom stop, didn't the bump stop/bottom stop just dictate the travel?

 

Ah... I see what you are saying now.

 

If the operational length or travel of the spring is longer than the travel of the shock assembly, the travel will be limited by the top and bottom stops on the shock assembly.

 

Got it.

 

Of course, we don't generally go around bottoming out or topping out the shock under normal riding (except under hard braking). And the more pre-load, the tighter the spring, the less travel for the same set of applied forces in actual practice.

 

If you are running with too much sag, like extra headroom for the suspension to float up, every upward rebound can push the suspension further. So, that is what I meant by 'potential for travel' (which is probably how they "rate" a shock), and then there is the actual amount of travel experienced while riding a given set of conditions and damping settings.

 

And this is what I was refering to when I said "at the expense of travel and geometry".

 

And now I get what you meant by moving the spring. You meant with more pre-load you will be running closer to top out. Hence, the shock assembly does not compress as far toward it's bottom out stop (assuming the spring is not pe-loaded very much).

 

Which brings me back around to stock springs vs racing springs which is where we started. While you may only be dialing 5-10 turns or threads for the proper spring, like I mentioned before, Johnny sportbike rider cranking a couple inches (or more if he adjusts for riding two up) on his stock spring is cranking that spring down WAY more.

 

And this is where I've been thinking. Of course, for your Penske, the pre-load factor was negligible ... which was my whole point and what I've been on about since page one.

 

 

Soo ... what about forks?

[racer]

So, what defines the travel of the forks?

[racer]

Let's say we set the sag at 1.5 inches, then we ride down the road and hit a bump that causes the suspension to compress 1 inch from the sag point and then return to the sag point. Then we crank down on the preload so that we get only 1 inch of sag, then we go out and hit the same bump at the same speed. Once again, the suspension will compress 1 inch FROM THE SAG POINT. Same bump, same speed, same spring, same starting weight on the spring thus same amount of starting spring compression, it will react the same UNLESS we reach top out or bottom out. Preload is just going to make the whole 1 inch of suspension movement resulting from that bump happen at a different portion of the available travel, or in other words, the bike will sit higher or lower as it rolls over that bump. In the context of this scenario it's perhaps not even accurate to call it "stiffer" or "softer" as is commonly said. So far no one has given me any actual mechanical explanation as to how this description is incorrect.

 

"'it will react the same UNLESS we reach top out or bottom out. "

 

So, setting sag would seem to be mostly about staying between the top out and bottom out for the rear shock, in the middle of the range so to speak. And, setting greater sag for street riding gives more effective travel range before topping out ... at increased risk of bottoming out?

 

 

As for a mechanical explanation of saying the spring is harder or softer, I would say it is a descriptive expression based on the fact that the spring is compressed more under preload. The more compressed a spring is, the more force it will take to compress it further. It is that simple. Starting from top out, x amount of force will compress the spring to the same point no matter the preload, but, with more distance traveled. So, though, while in riding mode, we won't compress the spring any further due to pre-load or experience a "harder" ride, we will alter the potential/effective travel from top out (and geometry of course).

 

 

 

 

In the case of front end dive under braking, relevant to this thread, if we increase front preload to reduce sag by .5 inches then repeat the hard braking in exactly the same way, then for all practical purposes the front end will have .5 inches less dive. The front end will sit .5 inches higher during the braking and all other times except when bottom out or top out is reached. Changing out the spring could make a much bigger difference, and would allow me to have less brake dive without reducing sag, which is why like you said, it is a much better option - as long as we don't decrease the fork's mid-corner performance.

 

So what defines or limits the bottom out in a standard fork?

 

Spring binding? Or will the airspace reach a critical compression that will prevent further compression before the spring bottoms out?

[racer]

Here's a decent overveiw of forks: http://www.motorcyclecruiser.com/tech/fork...logy/index.html

[racer]

*crickets chirping*

[harnois]

And now I get what you meant by moving the spring. You meant with more pre-load you will be running closer to top out. Hence, the shock assembly does not compress as far toward it's bottom out stop (assuming the spring is not pe-loaded very much).

So, setting sag would seem to be mostly about staying between the top out and bottom out for the rear shock, in the middle of the range so to speak. And, setting greater sag for street riding gives more effective travel range before topping out ... at increased risk of bottoming out?

Now we are definitely barking up the same tree.

 

As for a mechanical explanation of saying the spring is harder or softer, I would say it is a descriptive expression based on the fact that the spring is compressed more under preload. The more compressed a spring is, the more force it will take to compress it further. It is that simple. Starting from top out, x amount of force will compress the spring to the same point no matter the preload, but, with more distance traveled. So, though, while in riding mode, we won't compress the spring any further due to pre-load or experience a "harder" ride, we will alter the potential/effective travel from top out (and geometry of course).

 

Right, "from top out." If we crank down the preload it is "harder" in that it will take more force to move the suspension off the top out stop and it will sag less under a given amount of weight. So what I'm say'n is that since we do not spend the majority of our riding time topped out, it is not very useful to think of preload adjustment as making anything "harder" or "softer." As long as our suspension is floating freely, not topped out or bottomed out, as it should be most of the time, we are not going to make the suspension feel harder or softer by mess'n with preload. Preload will just make it float closer to top out or closer to bottom out. It's more useful IMO to think of the preload as nothing more than a sag adjustment. Although we do of course have to keep in mind that adjusting sag may have some effects on geometry.

 

Before I ever opened this thread I realized I could possibly solve the wiggle by cranking down on front preload. I might have even tried it while at the track - not sure, can't remember. If I did try it, it didn't work. And I also knew that cranking down on the prelaod was going to make the front end of the bike sit higher all the time, which theoretically would slow down the steering, hense "at the expense of geometry." However I'm not sure I thought about how going to stiffer front springs would get me less brake dive and still keep the better steering geometry.

[harnois]

So what defines or limits the bottom out in a standard fork?

 

Spring binding? Or will the airspace reach a critical compression that will prevent further compression before the spring bottoms out?

 

Seems to me forks are all the same story, except that perhaps we have the option of making a minor adjustment to the oil level to effectively create a more progressive spring rate. Forks are also designed so that they can hit the bottom out stop before the spring itself bottoms out. If the air in the forks becomes so compressed that it effectively becomes the bottom out stop, then some joker put too much oil in there.

[racer]

Here's a decent overveiw of forks: http://www.motorcyclecruiser.com/tech/fork...logy/index.html

 

 

Actually, while highlighting the difference between damper rod and cartridge fork dampers ok, that link wasn't really such a good operational "overview" of forks, in general. Upon further research, those illustrations leave a bit to be deisred IMO. For instance, it isn't clear in their damper rod model illustration that the top of the damping rod/resevoir (that acts as the bottom spring seat) remains stationary wrt the bottom tube while the upper tube slides past it. I found a better illustration here from the Gold Tech Emulator site:

 

http://www.sportrider.com/tech/146_9502_tech/photo_03.html

 

and this for the basic damper rod model:

 

http://www.sportrider.com/tech/146_9502_tech/photo_01.html

 

Much better with all the little arrows indicating oil flow, clear labels and better quality detailed drawings.

 

I've got a much better understanding of all of it now. Also thanks to your input harnois.